<!DOCTYPE html>
<html lang="en">
<head>
	<meta charset="UTF-8">
	<title>Document</title>
	<style>
		*{padding: 0; margin: 0}
		#main{width: 300px; height: 300px; margin: 0 auto; border:1px solid #ccc}
		#main .box{width: 100%; height: 100%; background: #ccc}
		#main:hover .box{animation: move 2s infinite}
		@keyframes move{
			0%{border-radius: 0;}
			50%{border-radius: 30%;}
			100%{border-radius: 50%;}
		}
	</style>
</head>
<body>
	<div id="main">
		<div class="box"></div>
	</div>
</body>
</html>